server: Fix panic when the server browser error handler tried to use a config in a state of flux

I tried and failed to create a test for this, but I have tested this manually.,

Fixes #14470

commands/commandeer.go

@@ -331,6 +331,9 @@ func (r *rootCommand) ConfigFromProvider(key configKey, cfg config.Provider) (*c
 }
 
 func (r *rootCommand) HugFromConfig(conf *commonConfig) (*hugolib.HugoSites, error) {
+	if conf == nil {
+		return nil, fmt.Errorf("conf must be set")
+	}
 	k := configKey{counter: r.configVersionID.Load()}
 	h, _, err := r.hugoSites.GetOrCreate(k, func(key configKey) (*hugolib.HugoSites, error) {
 		depsCfg := r.newDepsConfig(conf)

commands/hugobuilder.go

@@ -56,8 +56,9 @@ import (
 type hugoBuilder struct {
 	r *rootCommand
 
-	confmu sync.Mutex
-	conf   *commonConfig
+	confmu  sync.Mutex
+	confOld *commonConfig
+	conf    *commonConfig
 
 	// May be nil.
 	s *serverCommand
@@ -94,6 +95,27 @@ func (c *hugoBuilder) withConf(fn func(conf *commonConfig)) {
 	fn(c.conf)
 }
 
+func (c *hugoBuilder) withConfOrOldConf(fn func(conf *commonConfig)) {
+	c.confmu.Lock()
+	defer c.confmu.Unlock()
+	if c.conf != nil {
+		fn(c.conf)
+	} else if c.confOld != nil {
+		fn(c.confOld)
+	}
+}
+
+func (c *hugoBuilder) withConfOrOldConfE(fn func(conf *commonConfig) error) error {
+	c.confmu.Lock()
+	defer c.confmu.Unlock()
+	if c.conf != nil {
+		return fn(c.conf)
+	} else if c.confOld != nil {
+		return fn(c.confOld)
+	}
+	return errConfigNotSet
+}
+
 type hugoBuilderErrState struct {
 	mu       sync.Mutex
 	paused   bool
@@ -1095,6 +1117,7 @@ func (c *hugoBuilder) loadConfig(cd *simplecobra.Commandeer, running bool) error
 	}
 
 	c.conf = conf
+	c.confOld = conf
 	if c.onConfigLoaded != nil {
 		if err := c.onConfigLoaded(false); err != nil {
 			return err
@@ -1158,8 +1181,9 @@ func (c *hugoBuilder) reloadConfig() error {
 	c.r.resetLogs()
 	c.r.configVersionID.Add(1)
 
-	if err := c.withConfE(func(conf *commonConfig) error {
+	if err := c.withConfOrOldConfE(func(conf *commonConfig) error {
 		oldConf := conf
+		c.conf = nil
 		newConf, err := c.r.ConfigFromConfig(configKey{counter: c.r.configVersionID.Load()}, conf)
 		if err != nil {
 			return err

commands/server.go

@@ -282,7 +282,7 @@ func (f *fileServer) createEndpoint(i int) (*http.ServeMux, net.Listener, string
 					}
 
 					port = 1313
-					f.c.withConf(func(conf *commonConfig) {
+					f.c.withConfOrOldConf(func(conf *commonConfig) {
 						if lrport := conf.configs.GetFirstLanguageConfig().BaseURLLiveReload().Port(); lrport != 0 {
 							port = lrport
 						}

common/loggers/logger.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/bep/logg"
@@ -123,8 +124,11 @@ func New(opts Options) Logger {
 	)
 
 	l := logger.WithLevel(opts.Level)
+	logMu := &sync.Mutex{}
 
 	reset := func() {
+		logMu.Lock()
+		defer logMu.Unlock()
 		logCounters.mu.Lock()
 		defer logCounters.mu.Unlock()
 		logCounters.counters = make(map[logg.Level]int)
@@ -135,6 +139,7 @@ func New(opts Options) Logger {
 	}
 
 	return &logAdapter{
+		mu:          logMu,
 		logCounters: logCounters,
 		errors:      errorsw,
 		reset:       reset,
@@ -204,6 +209,7 @@ type Logger interface {
 }
 
 type logAdapter struct {
+	mu          *sync.Mutex
 	logCounters *logLevelCounter
 	errors      *strings.Builder
 	reset       func()
@@ -328,6 +334,8 @@ func (l *logAdapter) Errorln(v ...any) {
 }
 
 func (l *logAdapter) Errors() string {
+	l.mu.Lock()
+	defer l.mu.Unlock()
 	return l.errors.String()
 }
 

main_test.go

@@ -241,7 +241,9 @@ var commonTestScriptsParam = testscript.Params{
 			if err != nil {
 				ts.Fatalf("failed to read file %v", err)
 			}
-			newContent := bytes.Replace(oldContent, []byte(args[1]), []byte(args[2]), -1)
+			old, new := args[1], args[2]
+
+			newContent := bytes.Replace(oldContent, []byte(old), []byte(new), -1)
 			err = os.WriteFile(filename, newContent, 0o644)
 			if err != nil {
 				ts.Fatalf("failed to write file: %v", err)